Protests came in response to Thailand's Constitutional Court ruling that says demands for reforms to the monarchy are unconstitutional.
Thousands of Thais have taken to the streets of the country’s capital Bangkok demanding reforms of the monarchy.
Protesters marched on Sunday against rows of riot police behind shields, waving placards that read "No absolute monarchy" and "Reform is not abolition".
"The king's increased powers in recent years are pulling Thailand away from democracy and back to absolute monarchy," a protester read in a statement after the demonstration reached the German embassy in Bangkok.
"This is a fight to insist that this country must be ruled by a system in which everyone is equal."
Sunday's protest came in response to the Constitutional Court's ruling last week that a call for reforms to the monarchy by three protest leaders in August last year was unconstitutional and designed to topple the institution.
"The word 'reform' is not equivalent to abolition," said protester Peeyawith Ploysuwan, 25.
"You (authorities) only want to do the things you desire and see people with opposing views as bad guys... If society continues like this, how can we move forward?"
Three protesters have been wounded, a police spokesman said, adding that the incidents are being investigated.
Protests break long-lived taboos
The youth-led protests that began last year seeking removal of Prime Minister Prayuth Chan-ocha, 66, a former coup leader, have become the biggest challenge in decades to the monarchy which is constitutionally enshrined to be held in "revered worship".
Protesters also marched to the German embassy in October last year to urge Germany to investigate whether King Maha Vajiralongkorn, who spends much of his time in the country, was conducting state business from there.
The protests have broken longstanding taboos in Thailand, whose lese majeste law sets jail terms of up to 15 years for anyone convicted of defaming the monarchy.
Since the protests began, at least 157 people have been charged under the law, according to records compiled by the Thai Lawyers for Human Rights group.